Estuarine Front Research Articles

Mobile deltaic and continental shelf muds as suboxic, fluidized bed reactors

The oft-cited general correlation between net sediment accumulation and preservation of organic matter, while revealing in many ways, can be a misleading indicator of general elemental cycling processes and controls on storage of biogenic material at the continental-ocean boundary. Deltaic environments are characterized by the highest rates of net sedimentation and are the single most important class of depocenters on Earth. Available data indicate that sedimentary organic C (C org ) of both terrestrial and marine origin is efficiently decomposed in deltaic areas, with decomposition percentages reaching ≥70% and ≥90%, respectively, the latter percentage (marine) being quite comparable to deep-sea, low sedimentation environments. Despite high primary productivity associated with most deltas and evidence of substantial deposition of fresh planktonic debris, patterns of SO 4 = reduction indicate that the reactivity of organic material being buried is low, and that a larger proportion of C org is often degraded compared to other marine deposits of similar net accumulation rate. As indicated by properties of the surficial Amazon delta and downdrift coastal region of northeast South America (∼1600-km extent), the primary reasons for efficient remineralization are related to intense and massive physical reworking of sediment associated with estuarine fronts , upwelling, tidal oscillation, and wind-driven waves. Fluid muds and mobile surface material cause the seafloor and continental boundary to act as a massive, suboxic, fluidized bed reactor dominated in some cases by bacterial rather than macrofaunal biomass. Reoxidation, repetitive redox successions, metabolite exchange, and continual mixing-in of fresh planktonic debris with refractory terrestrial components, result in an efficient decomposition system largely decoupled from net accumulation. Similar processes occur on smaller scales in most estuarine-shelf systems, but appear to be most dramatically expressed off the major rivers forming deltas.

Estuarine fronts as conduits for larval transport:hydrodynamics and spatial distribution of Dungeness crab postlarvae

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 164:73-82 (1998) - doi:10.3354/meps164073 Estuarine fronts as conduits for larval transport: hydrodynamics and spatial distribution of Dungeness crab postlarvae David B. Eggleston1,*, David A. Armstrong2, Ward E. Elis1, William S. Patton2 1Department of Marine, Earth and Atmospheric Sciences, North Carolina State University, Raleigh, North Carolina 27695-8202, USA 2University of Washington, School of Fisheries, Box 357980, Seattle, Washington 98195, USA *E-mail: eggleston@ncsu.edu Frontal zones are common hydrographic features in estuaries throughout the world, yet there is no consensus on the importance of fronts to larval transport processes. Frontal circulation may act as a 'barrier' to larval transport since it constrains cross-frontal flows, leading to strong along-frontal flows, which in turn may serve as a type of 'larval conduit' by funneling larvae collected at the front to settlement locations. We examined the larval conduit hypothesis by characterizing hydrodynamic features of apparent frontal structure in Grays Harbor estuary (Washington, USA) and quantifying the effects of frontal structure on the distribution and transport potential of Dungeness crab Cancer magister megalopae during spring-time recruitment pulses. CTD casts made perpendicular to fronts indicated that frontal structure was characteristic of buoyant plume and axial convergent fronts at the bay mouth and at a subtidal channel site within the estuary, respectively. Surface drifters deployed seaward of a front at the bay mouth during early flood tide were advected into the front, demonstrating convergent circulation. Surface drifters remained within buoyant plume fronts, which moved up-estuary during flood tide at 0.4 m s-1, and were then entrained in axial convergent fronts. Drifters moved an average of 10 km up-estuary during a single flood tide; the direction that drifters tracked corresponded to prevailing winds. The mean concentration of megalopae, as measured with neuston nets, was significantly higher in fronts than in adjacent water, 20 to 30 m upstream of the front, irrespective of year or location within the estuary. Paired plankton nets deployed at the bay mouth during flood tide indicated that the mean concentration and flux of megalopae was higher at the surface than at 3 to 4 m depth; this trend was significant for megalopal flux. Variable wind stress during the recruitment period of Dungeness crab, combined with the suggested effect of fronts serving as larval conduits whose direction of transport is influenced by prevailing winds, could significantly influence spatial variation in postlarval supply and subsequent settlement within the estuary. Cancer magister · Drifters · Dungeness crab · Estuarine hydrodynamics · Fronts · Larval transport · Recruitment Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 164. Publication date: April 09, 1998 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1998 Inter-Research.

The Influence of Tidal Phase on Patterns of Ichthyoplankton Abundance in the Vicinity of an Estuarine Front, Botany Bay, Australia

The influence of oceanography on abundance patterns of ichthyoplankton was compared at different phases of the tide in the vicinity of an estuarine front in Botany Bay, Australia. Fronts were observed at high, ebb, low and flood tides, but the most conspicuous colour discontinuities were found on ebb and flood tides. Greatest differences in water density were found across the frontal region on ebb tides. Because rainfall was low, density differences were primarily due to differences in temperature. There was strong advection of drogues into fronts and along frontal margins on ebb and flood tides. Thus, particles are ‘ jetted ’ along fronts. Total ichthyoplankton was always greatest in fronts on low tides (by 1·5–4 times). Rank abundance of ichthyoplankton in the plume, front and ocean, at other phases of the tide (flood, high, ebb), varied among days. Great differences in the composition of ichthyoplankton were found between the plume, front and ocean at all phases of the tide, but differences were most clear on ebb and low tides. Some types of fish were always most abundant in the front (e.g. Exocoetidae), or the plume and front (e.g. Gobiidae, Pleuronectidae, Hemiramphidae, Blenniidae), regardless of the state of the tide. Some pelagic juveniles were only found in tows that contained drift algae, especially in the front. The ‘ jetting ’ of larvae along topographically stable fronts on flood tides may influence spatial patterns of recruitment in bays and estuaries. The intensity of differences in water density across the frontal region, between the plume and ocean, did not explain the relative abundance patterns of fish. The authors argue that a combination of physical advection, responses by ichthyoplankton to the biological attributes of fronts (e.g. abundance of food), lag effects (i.e. response of plankton to aggregation), and differences in larval supply (from spawning) were responsible for patterns of abundance in different water masses at scales of less than 15 km.

Photosynthetic and bacterial production gradients in a larval fish nursery:the St. Lawrence River transition zone

Phytoplankton biomass, primary production and bacterial abundance and production were measured across the freshwater-saltwater transition zone of the St. Lawrence River [Canada) during the seasonal period of maximum concentrations of fish larvae and macrozooplankton. The estuarine front was characterized by steep gradients in b~olog~cal s well as physical properties. Maximum turbidity and high phytoplankton biomass occurred in the well-mixed, low salinity (0.2 to 4 psu) region immediately upstream of the salt wedge. Peak zooplankton and ichthyoplankton biomass occurred within and slightly downstream of this frontal region The hypothesis that lower food chain processes were controlled exclusively by allochthonous carbon and bacterial heterotrophy was not supported. Photosynthetic rates per unit chlorophyll a [chl a) remained high across the freshwater-saltwater transition and the low light penetration was offset by a shallow mean depth of mixing. Bacterial concentrations and activity remained relatively constant across the transition, while chl a declined sharply downstream of the front, consistent with grazing losses. Photosynthesis contributed 34 to 66% of the total production (bacteria + phytoplankton). Freshwater phytoplankton advected from upstream contributed another 20 to 30%. These first-order estimates underscore the combined importance of photosynthesis plus bacterial processes within the downstream food web of large river ecosystems.

Seasonal Evolution of Microplanktonic Communities in the Estuarine Front Ecosystem of the Rhône River Plume (North-western Mediterranean Sea)

Seasonal evolution of microplanktonic communities was examined throughout the water column of the Rhône River outflow plume. Because of the lack of tide, the water column in the plume area corresponds to a three-layered system: an upper freshwater layer (plume layer) overlying Mediterranean seawater (seawater layer) and an intermediate layer (frontal layer) corresponding to the discontinuity. The three layers displayed different compositions of planktonic community. Except for bacteria, which were more abundant in the plume water during spring and summer, microorganisms were most abundant in the frontal water. Seasonal analysis showed a succession of microbial populations: primary producers (phytoplankton, autoflagellates) peaked in spring while heterotrophic organisms (bacteria, heteroflagellates) were more abundant in summer. In winter, the primary producers were cyanobacteria. Bacterial density and production were dependent upon the combined influence of temperature variation and nutrient concentration. The highest values of bacterial production were observed in the frontal layer. They were about 25 μg Cl -1 day -1, that is, respectively, two and three times more than the bacterial production in the freshwater and the seawater layers. Mortality rates were highest in the plume water (0·51 to 0·57 day -1), and only 60-70% of the mortality was due to the grazing. In the plume layer, the growth rate was twice in the fall of that in spring (1·34 against 0·51 ). The average growth rate in the frontal layer (1·57 day -1) was highest of the three layers and was coupled with the lowest mortality rate (0·22 day -1). In the frontal layer, the flagellate carbon requirement was only one tenth of the bacterial carbon production.

Estuarine Fronts: How Important Are They?

Estuarine fronts have a role in estuarine circulation, productivity, sediment dynamics, and water quality. Despite their importance, our understanding is rather modest. While some insight can be drawn from studies of oceanic fronts, the shorter time scales of estuarine fronts often exclude the role of Coriolis forcing and geostrophy from the hydrodynamics and exclude the prospect of physiological adaptation from ecosystem dynamics. Conservative processes and rapid nonconservative processes like aggregation, settling, and behavioral responses are expected to be most important at estuarine fronts. Although generally short-lived, the recurrent periodic nature of estuarine fronts does include longer time scales and some attention should be given to the importance of this recurrence to longer time scale ecological responses.

Surface Fronts in Estuaries: A Review

Recent advances in the understanding of the physics of estuarine fronts (small-scale regions of significant changes in hydrographic variables) are reviewed. These fronts may be classified in three groups—tidal mixing fronts, plume fronts, and shear fronts—on the basis of the mechanism dominating the local rate of change in the horizontal density gradient. These classes are not independent, however, since it appears that both tidal mixing fronts and shear fronts can produce buoyancy-driven flow structures that propagate away from the generation region as plume fronts. Field observations of typical examples of each class are presented and laboratory experiments and theoretical models which isolate the important physical processes are summarized. Some important unresolved questions are identified and suggestions for further research are offered.

Numerical Study on Frontal Phelloffiena Observed in Osaka Bay

The behaviours of the estuarine front and the tidal front, which are often observed in Osaka Bay, are examined by using a three-dimensional baroclinic flow model. In case of the estuarine front, buoyancy and earth's rotation are the main force to lead a strong geostrophic along-front jet. A tidal front appears along the 20 m-depth contour, where stratified waters at the inner bay and vertically well-mixed ones through straits meet with each other. Discontineous in density across the front is computed to be σt=2.5 at the sea surface.

Phytoplankton dynamics in a coastal jet frontal region

We studied the physiological status and turbulent transport of phytoplankton across the quasi-permanent 150 km long estuarine front separating the Gaspe current from the Anticosti gyre (NW Gulf of St. Lawrence, Canada). In early June, chl a concentrations reached 35 pg 1-' in the current, but remained < 1 pg 1-' in the gyre. At that time of maximum river runoff and horizontal shear stress, the phytoplankton biomass developing in the current was rapidly transported across the front by turbulent mixing. The strong correlation between chl a and salinity (r = -0.96) lndcated that the rate of crossfrontal dilution far exceeded the rate of phytoplankton production. The N : C and C .ch l a ratios increased and decreased, respectively, with cross-frontal mixlng as nutrient-deficient cells from the Gaspe current experienced higher nutrient concentrations and a deepening of the mixed layer in the front. As freshwater runoff and frontal mixing decreased in summer, phytoplankton biomass accumulated in the density front and at the base of the jet current, where low advection and sufficient light and nutrient conditions prevailed. Sporadic upwelling along the front contributed an estimated 5 and 9 O/O of the total seaward flux of nitrate and silicate, respectively. Thus, nutrients from the drainage basin of the St. Lawrence, rather than the frontal upwelling of deep water, would b e responsible for the bulk of the inorganic nutrient flux from the NW Gulf of St. Lawrence to the southern Gulf and the Scotian Shelf.

Practical Application of Theory for Tidal‐Intrusion Fronts

A simple theoretical model was applied to interpret the characteristics of an estuarine front that forms at the early stage of each flood tide off Newport News Point, in the lower James River, Virginia. The observed depths of diving of denser water at the front, and the upriver movement of the front are explained theoretically. The construction of a man‐made island to facilitate port expansion was proposed for downriver of the front. Its effect on the flood current approaching the front is quantified with a vertically averaged two‐dimensional numerical model. The theory is used to predict the impact of the proposed island on the frontal characteristics, particularly with respect to the entrainment and transport of oyster larvae to the lower portion of the water column, where net transport is upriver toward seed‐oyster beds. Applications of the front theory, results of the numerical model, and oyster‐larvae studies indicate that the transport capacity of the front would be markedly reduced by island construction at the proposed sites.

Ebb-Tidal Fronts in Charleston Harbor, South Carolina: Physical and Biological Characteristics

Surface accumulations of foam and flotsam as well as sharp salinity, density, turbidity gradients and regions of acoustic scatter were characteristic of ebb-tidal fronts in Charleston Harbor, South Carolina. Surface convergence velocities at these fronts averaged 0.06 m s−1 into the front at an angle of 30° to 60° with respect to the frontal axis, indicating along-front transport during the ebb. These fronts are tidally-induced, forming on the late flood and ebb along the interfaces of water masses. Horizontal and vertical measurements of density revealed that the upper harbor fronts form along the margin of a freshwater lens produced by riverine input. The hypothesis that these frontal zones have higher densities of phytoplankton and zooplankton than adjacent water masses was tested using chlorophylla measurements and net collections. The fronts did not demonstrate any significant accumulations of phytoplankton or zooplankton during the ebb tide. The results of this study suggest that the physical characteristics of ebb-tidal estuarine fronts in Charleston Harbor are periodic in nature and may indirectly affect plankton transport in this coastal plain estuary.

The formation of longitudinal fronts in a coastal plain estuary

Longitudinal estuarine fronts, aligned parallel to the axis of the estuary, are characteristic of the York River. Their time scale for genesis and decay is of the order of hours; they are usually located at the inner edge of the shoals or over the main channel. Field measurements have shown marked intratidal differences in density and current speeds across this estuary. These can be correlated with changes in depth and the strength of the advective processes. Differential advection between the channel and shoal regions, when acting upon a constant longitudinal density gradient, is shown to be of sufficient magnitude to generate strong lateral density gradients, and thus fronts, at particular times within the tidal cycle.

The dynamics of a bathymetrically arrested estuarine front

The mixing and circulation associated with a bathymetrically arrested estuarine front was studied using hydrographic and current data. A quasisteady front, exhibiting strongly convergent surface flows, is formed along the steeply sloping inner margins of the flood tide delta during each semidiurnal tide cycle. This front separates the brackish ambient water within a deep estuarine basin from the incoming oceanic tidal water. The position of the front is dependent on the local water depth and the difference in density between the two water masses. Beneath the surface there is an inclined frontal interface where static stability is very low and vertical mixing intense. A vertically integrated horizontal momentum equation was derived for flow in the upper layer and an estimate made as to the value of the associated entrainment coefficient.

Delineation of estuarine fronts in the German Bight using airborne laser-induced water Raman backscatter and fluorescence of water column constituents

The acquisition and application of airborne laser induced emission spectra from German Bight water during the 1979 MARSEN experiment is detailed for the synoptic location of estuarine fronts. The NASA Airborne Oceanographic Lidar (AOL) was operated in the fluorosensing mode. A nitrogen laser transmitter at 337.1 nm was used to stimulate the water column to obtain Gelbstoff or organic material fluorescence spectra together with water Raman backscatter. Maps showing the location and relative strength of estuarine fronts are presented. The distribution of the fronts indicates that mixing within the German Bight takes place across a relatively large area. Reasonable agreement between the patterns observed by the AOL and published results are obtained. The limitations and constraints of this technique are indicated and improvements to the AOL fluorosensor are discussed with respect to future ocean mapping applications.

Estuarine Front Formation and Propagation

Front conditions associated with estuarine saline wedges are investigated both theoretically and experimentally. It is deduced theoretically that two distinct front types should exist in a salt wedge estuary, namely wave fronts and St. Venant fronts. When the salt wedge is advancing in its ambient fluid on the flood tide a wave front exists characterized by a vertical wall of water. On the ebb flow however, when the salt wedge is retreating in its ambient fluid a St. Venant front should occur characterized by zero depth at the front. Experiments confirmed that these two front types do occur, but due to stability constraints, their frontal flow conditions are different from that predicted by theory. It is deduced that the fronts of all salt wedges effectively end at the critical flow section and that this should be the front criterion upon which a mathematical model of a salt wedge estuary is based.

Remote sensing of coastal fronts and their effects on oil dispersion

The use of remote sensing techniques to determine the properties of coastal and estuarine fronts, which represent regions of discontinuities and high gradients in ocean physical parameters such as velocity and density, and to assess the influence of such fronts on oil pollutants is discussed. Results of an aircraft and boat verification study of an oil drift and spread model in Delaware Bay are indicated which illustrate the tendency of oil slicks to be attracted to frontal regions, where a denser fluid underlies a lighter fluid giving rise to an inclined interface with convergence zones. Landsat imagery of the bay acquired in order to incorporate frontal information into the interactive computer model is then presented which allows the locations of coastal fronts to be charted throughout a tidal cycle. It is noted that satellite observations of flood-associated fronts on the New Jersey side of the bay and ebb-associated fronts on the Delaware side agree with boat measurements and model predictions, and that the remote tracking of fronts by aircraft and satellites will aid in oil slick clean-up operations.

Trace metal enhancement in the biotic and abiotic components of an estuarine tidal front

A frontal system of lower Delaware Bay was investigated to illustrate the role that such tidal fronts can have in concentrating trace metals and serving as a means for introducing trace metals into the trophic hierarchy of estuaries. This frontal system typically develops on ebb tide and can extend at least 32 km parallel to the central axis of Delaware Bay. In relation to samples collected on either side of the front, chlorophyll a concentrations and concentrations of selected trace metals in zooplankton, particulates, and the dissolved fraction were significantly higher in samples collected at the front. Seasonally, concentrations of cadmium and copper in all fractions were highest during the summer or fall. Differences in taxonomic composition of zooplankton assemblages, as well as differences in total numbers of organisms across the front, suggested advective transport by convergent water masses.

Reply [to “Comment on ‘Characteristics of a tide-induced estuarine front’ by R. Grant Ingram”

Reply [to “Comment on ‘Characteristics of a tide-induced estuarine front’ by R. Grant Ingram”

Comment on ‘Characteristics of a tide-induced estuarine front’ by R. Grant Ingram

Comment on ‘Characteristics of a tide-induced estuarine front’ by R. Grant Ingram

Characteristics of a tide-induced estuarine front

Characteristics of a dynamic front over the Ile Rouge Bank in the Saint Lawrence estuary are discussed. It is shown that an advancing front, exhibiting both a large density gradient and a convergent surface flow, is generated during each semidiurnal tide cycle. Frontal advance occurs just after high tide on the northern shore. During retreat, frontlike characteristics are not observed.